Add per-level wind atmosphere property

WIND directive in .lvl files sets a constant horizontal force (px/s^2)
that pushes entities, projectiles, and particles. Positive is rightward.

Wind is applied as acceleration in physics_update() (halved on ground),
directly to projectile and particle velocities, and as a gentle position
drift on flyers. Entities with gravity_scale=0 (drones, spacecraft) are
unaffected. Levels default to no wind when the directive is absent.

src/game/enemy.c

@@ -170,6 +170,13 @@ static void flyer_update(Entity *self, float dt, const Tilemap *map) {
     float bob_offset = sinf(fd->bob_timer * FLYER_BOB_SPD) * FLYER_BOB_AMP;
     body->pos.y = fd->base_y + bob_offset;
 
+    /* Wind drifts flyers (they bypass physics_update).
+     * Apply as gentle position offset matching first-frame physics. */
+    float wind = physics_get_wind();
+    if (wind != 0.0f) {
+        body->pos.x += wind * dt * dt;
+    }
+
     /* Chase player if in range */
     Entity *player = find_player(s_flyer_em);
     if (player && player->active && !(player->flags & ENTITY_DEAD)) {